The proposed research is directed toward the problem of the regulation of ammonia utilization and arginine and urea biosynthesis in mammalian systems, and the factors of mechanisms responsible for the coordination of urea synthesis with the other pathways of amino acid metabolism. To this end, the first enzymes of the urea pathway, carbamylphosphate synthetase and ornithine transcarbamylase, will be studied to determine their role in this process. The mechanism of catalysis and the regulatory properties of the highly purified enzymes are being investigated. The physicochemical properties of the two enzymes are being studied in order to establish the structural features which determine the active and inactive forms and macromolecular organization of the enzymes. This approach, based on structure, will provide a framework on which to interpret kinetic behavior, substrate binding, and interaction with metabolites, and to evaluate the significance of these factors in the regulation of ammonia utilization. The role of N-acetyl-L-glutamate as a key factor in this process will be explored, both in terms of its mechanism of action in the activation of carbamylphosphate synthetase as well as its biosynthesis.